1. Field of the Invention
The present invention pertains to a radio base station, a radio communication system, a communication control method, and a recording medium recording a communication control program, in which a plurality of radio base stations are connected in the system, and a radio communication terminal can switch the radio base stations for connection while moving.
2. Description of Related Art
Generally, in a wireless LAN (Local Area Network) using a wireless transmission medium, a radio communication terminal performs a switching operation (hand-over) for the radio base station to be connected, when a received signal level from the radio base station to the radio communication terminal is deteriorated.
In performing the hand-over, the radio base station of destination to be connected by switching is decided depending on a radio communication signal quality received by the radio communication terminal. The radio communication terminal measures a reception level of report information (beacon) or a data frame from the radio base station being currently connected, and a response confirmation packet to the data frame transmitted from the radio communication terminal, and a ratio of reception level to noise floor (hereinafter, SN ratio: Signal to Noise Ratio), and compares them with preset thresholds.
If the reception level or the SN ratio is below the preset threshold, the radio communication terminal transmits a retrieval packet for retrieving other peripheral radio base stations. And the radio communication terminal receives a retrieval response packet from each radio base station to make a table management, and compares a radio wave communication quality (reception level, SN ratio) from each radio base station. As a result of comparison, the radio communication terminal selects the radio base station having a higher reception level or a SN ratio and makes a connection request. That is, the radio base station only transmits a response packet to the retrieval packet from the radio communication terminal, whereby the radio communication terminal makes a decision for the connection destination based on the management and the comparison results.
As a relevant technique made by the present applicant, there is a system in which the radio base station periodically notifies the wireless resource information of peripheral radio base stations to the radio communication terminal. In this system, the radio communication terminal decides the radio base station of destination to be switched to by hand-over, using the notified wireless resource information, and notifies the decision to the radio base station. And the radio base station receiving a notification requests the radio base station of destination to be switched to, to secure a wireless resource (e.g., refer to patent document 1 (Japanese Patent Application Laid-Open No. 2003-348007).
Also, as a technique related to the present invention, there is a system in which in performing the hand-over, a wireless packet communication apparatus transmits to the radio base station of destination to be switched a request to send communication traffic volume information. In this system, a final judgment for switching over to the radio base station of admissible destination is made, based on the communication traffic volume information from the radio base station of destination to be switched to (e.g., refer to patent document 2 (Japanese Patent Application Laid-Open No. 2004-186953).
Also, as a relevant technique made by the present applicant, there is a system in which a mobile terminal unit transmits to the radio base station a message containing information on a resource amount required for a communication session. In this system, the radio base station transmits to the mobile terminal unit a message containing the radio base station selected as a destination candidate for transfer by hand-over and the information on the communication quality. And the mobile terminal unit receiving the message decides the radio base station of destination to be switched to by hand-over, based on the information of the transmitted message (e.g., refer to patent document 3 (Japanese Patent Application Laid-Open No. 2005-94806).
Next, the problems with those relevant techniques will be described below.
Firstly, in switching the radio base stations by hand-over in the above typical systems, if a situation of the radio base station of destination to be switched to is changed, the radio base station of destination to be switched to may refuse the connection.
If a reconnection request by switching is refused, it is required to retrieve the radio base station of destination to be switched to again, and transmit a connection request again, whereby it takes a lot of time to make the reconnection.
Also, to stabilize the communication, it is required to secure a service band at the radio frequency for use in the radio communication (hereinafter a radio band) in making the reconnection, whereby there is a fear that it possibly takes a lot of time to make the reconnection.
Herein, if the required radio band can not be secured, for example, the communication band required for a voice communication can not be assured, whereby a degradation in a speech communication quality or a disconnection may occur following the hand-over. Also, in a real time communication for a moving picture or the like, a failure such as disordering or freezing of an image may occur.
Also, in the above-cited patent documents 1 to 3, the radio communication terminal decides the radio base station of destination to be switched to by hand-over to have a increased reliability of radio communication. However, it is not considered to shorten a time taken to decide the radio base station of a destination candidate to be connected again when the reconnection request is refused in switching the radio base stations by hand-over.